What about on his side? I often put DD2 on her side because it helps her sleep better. Tummy worries me with the whole SIDS thing, in the day if you are there watching I wouldn't worry but at night being asleep I would worry.
I put DD on her side but make sure her arm isn't wrapped so she can't just roll over.
If he is on his back put his head sideways so if he does vomit he can't choke on it.